adding pydap package

[dgergel]

It would be great if we could add the pydap package: https://github.com/pydap

When using open_mfdataset with engine='pydap', this fails without having the pydap package installed, e.g.

obs = xr.open_mfdataset(fnames, engine='pydap', concat_dim='day').rename({'day': 'time'}).drop('crs').

[delgadom]

great! I'll add you as a contributor to this package. Could you create a pull request (can be a branch or fork - would probably prefer a branch so we can co-edit) with this package added to base-environment.yml?